Impact of strength and size of donors on the optoelectronic properties of D-π-A sensitizers

Article abstract of DOI:10.1039/c6ra01185c

A series of carbazole based sensitizers with either phenyl based donors (TBC, TMC, OMC, PC, TBR, TMR, OMR and PR) or aryl amine based donors (OMNC, CNC and HNC) as well as one without a donor group (CC) have been synthesized to understand the influence of the strength of the donor moiety on the optical, electrochemical and photovoltaic properties. Two different acceptor moieties such as cyano acrylic acid and rhodanine acetic acid were introduced and evaluated. Different substituents on the phenyl group have a significant impact on the light harvesting ability of the sensitizers. Among phenyl based donors, anisole based carbazole (OMC) shows the highest short circuit current (J_SC) of 4.96 mA cm^-2 with overall power conversion efficiency (PCE) of 2.69%. In the case of the sensitizers with aryl amine based donors, the increasing bulkiness of the donor group lead to increasing open circuit potential. Transient photocurrent and photovoltage measurements signify the importance of a bulky donor fragment in determining the open circuit potential of the dyes. Sensitizers with hexyloxy substituted phenyl amine as the donor group shows a J_SC of 6.84 mA cm^-2 with PCE of 3.33%. The overall investigation provides vital information about the influence of donor groups on the optoelectronic properties of the sensitizers for its photovoltaic applications.

Full text of DOI:10.1039/c6ra01185c

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Impact of strength and size of donors on the
optoelectronic properties of D–p–A sensitizers†
Cite this: RSC Adv., 2016, 6, 37347
a
b
b
b
J. Sivanadanam, P. Ganesan, Peng Gao, Md. K. Nazeeruddin, Alexei Emeline,cd
*
*
Detlef Bahnemanncd and R. Rajalingam
*
a
A series of carbazole based sensitizers with either phenyl based donors (TBC, TMC, OMC, PC, TBR, TMR,
OMR and PR) or aryl amine based donors (OMNC, CNC and HNC) as well as one without a donor group
(CC) have been synthesized to understand the influence of the strength of the donor moiety on the
optical, electrochemical and photovoltaic properties. Two different acceptor moieties such as cyano
acrylic acid and rhodanine acetic acid were introduced and evaluated. Different substituents on the
phenyl group have a significant impact on the light harvesting ability of the sensitizers. Among phenyl
based donors, anisole based carbazole (OMC) shows the highest short circuit current (JSC) of 4.96 mA
cmꢀ2 with overall power conversion efficiency (PCE) of 2.69%. In the case of the sensitizers with aryl
amine based donors, the increasing bulkiness of the donor group lead to increasing open circuit
potential. Transient photocurrent and photovoltage measurements signify the importance of a bulky
donor fragment in determining the open circuit potential of the dyes. Sensitizers with hexyloxy
substituted phenyl amine as the donor group shows a JSC of 6.84 mA cmꢀ2 with PCE of 3.33%. The
overall investigation provides vital information about the influence of donor groups on the
optoelectronic properties of the sensitizers for its photovoltaic applications.
Received 14th January 2016
Accepted 21st March 2016
DOI: 10.1039/c6ra01185c
www.rsc.org/advances
advantages like high molar extinction coefficient, structural
exibility, low cost, easier preparation and purication process.
1. Introduction
In the past years, sensitizers with various aromatic building
block such as coumarin,4 indoline,5 perylene,6 merocyanine,7
porphyrin,8 triarylamine9 and carbazole10 have been reported,
all of which have obtained power conversion efficiency in the
range of 5–9%.
Dye sensitized solar cells (DSSC) are one of the most promising
methods to convert solar energy to electrical energy at low cost
with high power conversion efficiency.1,2 DSSC comprises four
major components i.e., a semiconductor metal oxide, sensitizer,
redox active electrolyte and counter electrode. Sensitizers play
a major role in harvesting visible light radiations to achieve
high power conversion efficiency and have been intensively
studied. To date, dyes based on ruthenium complexes and
porphyrin sensitizers achieved maximum power conversion
efficiencies of 11 and 13%, respectively.1,3 Development of metal
free organic sensitizers has been increased vastly in this decade
due to the rare, expensive ruthenium metal and the difficulties
in purication process. Organic sensitizers however have
Carbazole is a well-known heterocyclic compound which has
unique optoelectronic properties, and has found applications in
hole transporting material in organic devices, electron donors
in solar cells and host material in electroluminescent
devices.11,12 Chemically, carbazole is easily functionalized at 3, 6
and 9 positions and covalently bonded to other molecular
compounds.13,14 Recently, Jianhua Su14 reported the D–p–A
molecules functionalized at the above positions showing the
efficiency up to 5.91%. Most recently, sensitizer (MK-2) based
on carbazole reported by Z. S. Wang and co-workers produced
a maximum power conversion efficiency of 8.3%.13 The classic
structural arrangement for organic dyes comprises the donor
fragment and acceptor fragment linked by the p spacer (D–p–
A). Among various types of donor groups utilized in the DSSC,
aryl amine based donor groups have been most popular options
achieving reasonable efficiencies.15 Increasing the twist angle
between the donor and p spacer will avoid the dye aggregation
that eventually assists in reducing the charge recombination
between the injected electrons and the oxidized dyes. Recently,
uorenes and dibenzo heterocycles such as carbazole were
aSchool of Chemistry, Bharathidasan University, Tiruchirappalli 620024, Tamilnadu,
India. E-mail: rrengas@gmail.com
bGroup for Molecular Engineering of Functional Materials, EPFL Valais Wallis, Rue de
l’industrie 17, CH-1951 Sion, Switzerland. E-mail: peng.gao@ep.ch; mdkhaja.
nazeeruddin@ep.ch
cLaboratory “Photoactive Nanocomposite Materials”, Saint-Petersburg State
University, Ulyanovskaya str. 1 Peterhof, Saint-Petersburg, 198504 Russia. E-mail:
detlef.bahnemann@spbu.ru
d“Photocatalysis and Nanotechnology”, Institut fuer Technische Chemie, Gottfried
Wilhelm Leibniz Universitaet Hannover, Callinstrasse 3, D-30167 Hannover,
Germany. E-mail: bahnemann@ic.uni-hannover.de
† Electronic supplementary information (ESI) available: Synthetic procedure of
carbazole intermediates and the characterization. See DOI: 10.1039/c6ra01185c
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 37347–37361 | 37347

View Article Online
RSC Advances
Paper
Scheme 1 Structures of carbazole sensitizers with phenyl based donors.
successfully employed as p spacer by gratifying the above short circuit current density JSC, open circuit potential VOC by
mentioned criteria.16 Further functionalization of different varying the substituents in the donor fragments were discussed.
donor moieties in the carbazole fragment is easy and conve- The potential of the sensitizers to convert the absorbed light in
nient that will aid in constructing the required donor–p– to current was analysed using IPCE measurements. Transient
acceptor structural arrangement. In this context, a complete photocurrent and photovoltage measurements were carried out
study on the impact of strength of donor groups in a simple D– to analyse the importance of bulky donor fragment in deter-
p–A sensitizers will unveil the role of the donor group in the mining the open circuit potential of the dyes.
light harvesting ability, electrochemical and photovoltaic
parameters.
2. Experimental part
Based on these points, in this paper we have synthesised
carbazole based sensitizers with/without donor groups of
different donating strength. All the new sensitizers were
successfully synthesised and characterized. The differences in
the strength of the donor fragments reected in the absorption,
emission, electrochemical properties of the carbazole sensi-
tizers. The variations in the photovoltaic parameters such as
Carbazole, 1-bromo octane, rhodanine acetic acid, (2,4,6-tri-
methylphenyl)boronic acid, 4-methoxyphenylboronic acid, 4-
phenylboronic acid and Pd(PPh3)4 were purchased from
Aldrich. 1,2-Dichloroethane, sodium hydroxide, potassium
iodide, potassium carbonate and 100–200 mesh silica gel were
from Merck chemicals. Phosphorous oxy chloride, cyano acetic
37348 | RSC Adv., 2016, 6, 37347–37361
This journal is © The Royal Society of Chemistry 2016

View Article Online
Paper
RSC Advances
Scheme 2 Structures of carbazole sensitizers with amine based donors.
Scheme 3 Synthetic pathways for the preparation of CBAI. Reagents: (1) carbazole, 1-bromo octane, NaOH, DMSO, RT. (2) CB, DMF, POCl3,
DCE, 90 ꢁC. (3) CBA, KI, KIO3, acetic acid, 80 ꢁC.
acid and ammonium acetate were obtained from Loba chem- electrode, the counter electrode was a platinum wire and the
icals. 4-tert-Butylphenylboronic acid and potassium iodate from reference electrode was a Ag/AgCl. Tetrabutylammonium hexa-
Spectrochem. All solvents were purchased from spectrometric uoro phosphate (TBAPF6, 0.1 M) was used as supporting
grade and used as received without further purication. 1H electrolyte in acetonitrile solution for phenyl based donors and
NMR and 13C NMR spectrum was recorded on a Bruker 400 in dichloromethane solutions for amine based donors. A pinch
MHz spectrometer in CDCl3-d or DMSO-d6 solvent with tetra- of ferrocene was added to each sample and ferrocene/
methylsilane as internal standard. Mass spectra were obtained ferrocenium (Fc/Fc+) was used as the internal standard. The
with Bruker daltonics, FT-ICR/APEX II, ESI mode. MALDI-TOF potential of the dyes vs. normal hydrogen electrode (NHE) were
mass spectra were recorded using Micromass Tof Spec 2E calibrated by using ferrocene (0.630 V vs. NHE) as the internal
instrument. High-resolution mass spectra were obtained at the standard.17 All calculations were carried out using Spartan'10
´
´ ´
Ecole Polytechnique Federale de Lausanne mass spectrometry Windows run on Microso Windows XP, Geometry optimiza-
laboratory (EPFL). tion, energy levels, and frontier molecular orbitals of the dyes'
Absorption spectra of dyes in chloroform solution and on HOMOs and LUMOs were calculated at the B3LYP/6-31G (d,p)
TiO2 lms were recorded using a JASCO 300 UV-Visible spec- level.
trometer. Fluorescence measurements were carried out using
A 450 W xenon lamp (Oriel, USA) was used as a light source to
Perkin Elmer LS 55 spectrouorimeter with 10 nm slit width study the current–voltage characteristics of the DSSC. The
and 200 nm minꢀ1 scan width was kept constant for all spectral output of the lamp was ltered using a Schott K113
¨
measurements. Electrochemical studies were examined by Tempax sunlight lter (Prazisions Glas & Optik GmbH, Ger-
cyclic voltammetry (SP-50 model potentiostat) using a three many) to reduce the mismatch between the simulated and
electrode system. The working electrode was a platinum actual solar spectrum to less than 2%. The Keithley model 2400
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 37347–37361 | 37349

View Article Online
RSC Advances
Paper
nm sized TiO2 particles was then printed on the FTO conduct-
ing glass and further coated with a 5 mm thick second layer of
light-scattering TiO2 particles (400 nm diameter, Catalysts &
Chemicals Ind. Co. Ltd (CCIC), HPW-400). Sintering was carried
out at 500 ꢁC for 15 min, which was gradually heated. The
working electrode was prepared by immersing the 13.0 mm (8.0
mm thick transparent layer + 5.0 mm thick scattering) TiO2 lm
into the dye solution for 12 h. To prepare the counter electrode,
Pt catalyst was deposited on cleaned FTO glass by coating with
a drop of H2PtCl6 solution (10 mM in 2-propanol solution) with
heat treatment at 400 ꢁC for 15 min. For the assembly of DSSCs,
the dye-containing TiO2 electrode and Pt counter electrode were
assembled into a sandwich-type cell and sealed with a hot-melt
gasket of 25 microns thickness made of the ionomer Surlyn
1702 (Dupont). Devices were completed by lling the electrolyte
through the pre-drilled holes in the counter electrodes and
nally the holes were sealed with a Surlyn sheet and a thin glass
cover by heating. A black mask (6 ꢂ 6 mm) was used in the
subsequent photovoltaic studies.
4. Synthesis and characterization of
carbazole based sensitizers
The complete synthetic procedures of all intermediate
compounds are available in ESI.† Three amine donor moieties
(11, 12 and 13) are synthesized according to the literature.19,20
Scheme 4 Synthetic routes for the preparation of phenyl donor based
carbazole sensitizers. Reagents: (4) CBAI, (4-tert-butylphenylboronic
acid (5) 2,4,6-trimethylphenyl)boronic acid (6), 4-methox-
yphenylboronic acid (7), 4-phenylboronic acid (8), Pd (PPh3)4, THF,
H2O, 70 ꢁC. (9) CBA, CTB, CTM, COMe and CPB, ammonium acetate,
acetic acid, 120 ꢁC.
4.1. General procedure for the preparation of target
compounds
To a mixture of corresponding aldehydes (1 mol ratio), cyano-
acetic acid or rhodanine acetic acid (1.5 mol ratio) and ammo-
nium acetate (1.95 mol ratio) in glacial acetic acid (5 ml) was
stirred at 80 ꢁC for 1 day. Aer completion of the reaction, cool
to RT followed by quench the mixture in ice water, a precipitate
was obtained. It was ltered and dried to afford the desired
product.
digital source meter (Keithley, USA) was used for data acquisi-
tion. The photo-active area of 0.16 cm2 was dened by a black
mask of 6 ꢂ 6 mm2. Incident photon-to-current conversion
efficiency measurements were measured using the mono
chromated visible photons, from Gemini-180 double mono-
chromator Jobin Yvon Ltd (UK), powered by a 300 W xenon light
source (ILC Technology, USA) superimposed on a 10 mW cmꢀ2
LED light. The monochromatic incident light was passed
through a chopper running at 2 Hz frequency and the on/off
ratio was measured by an operational amplier. Photovoltage
transients were observed by using a pump pulse generated by 4
red light emitting diodes controlled by a fast solid-state switch
with a white light bias. The pulse of red light with widths of 50
ms was incident on the photoanode side of the cell, and its
intensity was controlled to keep a suitably low level to generate
the exponential voltage decay where the charge recombination
rate constants are obtained directly from the exponential decay
rate.18
3-[6-(4-tert-Butyl-phenyl)-9-octyl-9H-carbazol-3-yl]-2-cyano-
acrylic acid (TBC). Product: yellow colored solid, yield: 71%.
1
ꢁ
Melting point: 244 C H NMR, (400 MHz, CDCl3-d): d ¼ 8.78
(s, 1H), 8.48 (s, 1H), 8.32 (s, 1H), 8.26 (d, J ¼ 8 Hz, 1H), 7.77 (d,
J ¼ 8.4 Hz, 1H), 7.64 (d, J ¼ 8 Hz, 2H), 7.52–7.46 (m, 4H), 4.31
(s, 2H), 1.89 (s, 2H), 1.39 (s, 9H), 1.33–1.25 (m, 10H), 0.86 (t, J
¼ 6.4 Hz, 3H) ppm. 13C NMR, (100 MHz, CDCl3-d): d ¼ 168.77,
157.21, 149.88, 143.91, 140.32, 138.42, 134.11, 129.58, 126.95,
126.30, 125.84, 123.66, 123.24, 122.47, 119.11, 116.77, 109.63,
109.50, 43.51, 34.54, 31.76, 31.43, 29.72, 29.29, 29.15, 28.95,
27.24, 22.60, 14.06 ppm. HRMS (ESI-TOF) m/z: calculated for
C
34H38N2O2 [M + H]+: 506.29, found: 507.3193.
{5-[6-(4-tert-Butyl-phenyl)-9-octyl-9H-carbazol-3-ylmethylene]-
4-oxo-2-thioxo-thiazolidin-3-yl}-acetic acid (TBR). Product:
orange colored solid, yield: 72%. Melting point: 254 ꢁC 1H NMR,
(400 MHz, DMSO-d6): d ¼ 8.53–8.47 (m, 2H), 7.98 (s, 1H), 7.74–
3. Device fabrication
A screen-printed double layer of nanocrystalline TiO2 particles 7.65 (m, 6H), 7.45 (d, J ¼ 8.4 Hz, 2H), 4.69 (s, 2H), 4.39 (s, 2H),
was used as the photoelectrode. The FTO glass plates were 1.73 (s, 2H), 1.28 (s, 9H), 1.20–1.10 (m, 10H), 0.74 (t, J ¼ 4.8 Hz,
ꢁ
immersed in a 40 mM aqueous TiCl4 solution at 70 C for 30 3H) ppm. HRMS (ESI-TOF) m/z: calculated for C36H40N2O3S2 [M +
min and washed with water and ethanol. A 8 mm thick lm of 20 H]+: 612.25, found: 613.2781.
37350 | RSC Adv., 2016, 6, 37347–37361
This journal is © The Royal Society of Chemistry 2016

View Article Online
Paper
RSC Advances
Scheme 5 Synthetic routes for the preparation of amine donor based carbazole sensitizers. Reagents: (10) CBAI, bis-(4-methoxy-phenyl)-amine
(11), bis-(4-hexyloxy-phenyl)-amine (12), bis(20,40-bis(hexyloxy)-[1,10-biphenyl]-4-yl)amine (13), Pd[P(t-Bu)3]2, sodium tert-butoxide, toluene, 80
ꢁC. (9) COMN, CCN and CHN, ammonium acetate, acetic acid, 120 ꢁC.
122.34, 121.47, 116.58, 109.60, 109.52, 96.36, 43.70, 31.79, 29.34,
29.19, 29.06, 27.34, 22.63, 21.10, 21.01, 14.10 ppm. HRMS (ESI-
TOF) m/z: calculated for C33H36N2O2 [M + H]+: 492.28, found:
493.2904.
{5-[9-Octyl-6-(2,4,6-trimethyl-phenyl)-9H-carbazol-3-ylmethy-
lene]-4-oxo-2-thioxo-thiazolidin-3-yl}-acetic acid (TMR). Product:
orange colored solid, yield: 72%. Melting point: 214 ꢁC 1H NMR,
(400 MHz, CDCl3-d): d ¼ 8.21 (s, 1H), 8.00 (s, 1H), 7.89 (s, 1H),
7.62 (d, J ¼ 8 Hz, 1H), 7.48 (d, J ¼ 8.8 Hz, 2H), 7.31 (d, J ¼ 8 Hz,
2-Cyano-3-[9-octyl-6-(2,4,6-trimethyl-phenyl)-9H-carbazol-3-yl]-
acrylic acid (TMC). Product: yellow colored solid, yield: 57%.
Melting point: 226 ꢁC 1H NMR, (400 MHz, CDCl3-d): d ¼ 8.75 (s,
1H), 8.46 (s, 1H), 8.26 (d, J ¼ 8.8 Hz, 1H), 7.91 (s, 1H), 7.49 (d, J ¼
7.6 Hz, 2H), 7.31 (d, J ¼ 8.3 Hz, 1H), 6.99 (s, 2H), 4.36 (t, J ¼ 7.2
Hz, 2H), 2.36 (s, 3H), 2.03 (s, 6H), 1.95 (t, J ¼ 7.2 Hz, 2H), 1.44–
1.25 (m, 10H), 0.87 (t, J ¼ 6 Hz, 3H) ppm. 13C NMR, (100 MHz,
CDCl3-d): d ¼ 168.99, 157.52, 144.02, 140.01, 138.96, 136.71,
136.48, 133.88, 129.60, 128.57, 128.15, 126.04, 123.71, 122.97,
Fig. 1 Normalised absorption spectrum of phenyl donor based carbazole dyes in CHCl3 solutions.
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 37347–37361 | 37351

View Article Online
RSC Advances
Paper
Table 1 Absorption, emission and electrochemical properties of phenyl donor based carbazole dyes
lmaxa (nm)/[3 ꢂ 105 (Mꢀ1 cmꢀ1)]
lmaxa (nm) on TiO2 lm
lemia (nm)
Eoxb (V) vs. NHE
E0–0c (eV) vs. NHE
Eox–E0–0 (V)
TBC
TMC
OMC
PC
416 (2.66)
413 (1.97)
418 (7.66)
414 (4.36)
408 (1.70)
456 (1.66)
453 (2.74)
457 (3.23)
454/1.86
399
371
404
400
403
371
371
371
371
530
500
545
518
480
537
523
553
530
1.15
1.58
1.39
1.54
1.53
1.44
1.44
1.30
1.45
2.71
2.79
2.66
2.74
2.85
2.52
2.56
2.49
2.54
ꢀ1.56
ꢀ1.21
ꢀ1.27
ꢀ1.19
ꢀ1.32
ꢀ1.08
ꢀ1.12
ꢀ1.19
ꢀ1.09
CC
TBR
TMR
OMR
PR
a
Absorption and emission spectrum of phenyl donor based carbazole dyes were measured in CHCl3 solutions. b Cyclic voltammograms of the rst
oxidation potential of the dyes were measured in acetonitrile solution containing 0.1 M TBAPF6 used as an supporting electrolyte with a scan rate of
100 mV sꢀ1 (working electrode: Pt, counter electrode: Pt wire, reference electrode: Ag/AgCl calibrated with ferrocene/ferrocenium (Fc/Fc+) as an
c
internal reference and converted to NHE by adding 630 mV). The E0–0 transition values was estimated from the onset of absorption and
emission spectra.
1H), 6.99 (s, 2H), 4.94 (s, 2H), 4.34 (t, J ¼ 7.2 Hz, 2H), 2.37 (s, 3H),
[5-(9-Octyl-6-phenyl-9H-carbazol-3-ylmethylene)-4-oxo-2-thioxo-
2.04 (s, 6H), 1.93 (p, J ¼ 7.2 Hz, 2H), 1.47–1.25 (m, 10H), 0.87 (t, J
thiazolidin-3-yl]-acetic acid (PR). Product: red colored solid, yield:
¼ 6 Hz, 3H) ppm. 13C NMR, (100 MHz, CDCl3-d): d ¼ 192.97,
171.36, 167.30, 142.13, 139.94, 139.08, 136.68, 136.52, 136.31,
133.33, 128.93, 128.34, 128.15, 124.29, 124.04, 123.88, 122.75,
121.28, 117.94, 109.74, 109.34, 44.41, 43.60, 31.79, 29.73, 29.35,
29.19, 29.07, 27.35, 22.63, 21.10, 21.03, 14.10 ppm. HRMS (ESI-
TOF) m/z: calculated for C35H38N2O3S2 [M + H]+: 598.23, found:
599.2596.
86%. Melting point: 92 ꢁC 1H NMR, (400 MHz, DMSO-d6): d ¼ 8.59
(s, 1H), 8.50 (s, 1H), 7.98 (s, 1H), 7.80–7.68 (m, 5H), 7.44 (t, J ¼ 7.2
Hz, 2H), 7.30 (t, J ¼ 7.2 Hz, 1H), 4.70 (s, 2H), 4.41 (t, J ¼ 6.8 Hz, 2H),
1.74 (t, J ¼ 6.4 Hz, 2H), 1.21–1.11 (m, 10H), 0.74 (t, J ¼ 6.4 Hz, 3H)
ppm. 13C NMR, (100 MHz, DMSO-d6): d ¼ 193.65, 167.78, 166.90,
142.41, 141.17, 140.71, 136.15, 132.97, 129.61, 129.32, 128.82,
127.27, 127.19, 127.15, 126.23, 124.75, 124.16, 123.72, 123.04,
119.47, 117.76, 111.08, 110.84, 55.32, 45.42, 43.15, 31.59, 29.12,
29.01, 28.95, 26.85, 22.43, 14.31 ppm. MS (ESI) m/z: calculated for
C32H32N2O3S2 [M ꢀ H]+: 556.19, found: 555.25.
2-Cyano-3-[6-(4-methoxy-phenyl)-9-octyl-9H-carbazol-3-yl]-acrylic
acid (OMC). Product: dark yellow colored solid, yield: 93%. Melting
point: 182 ꢁC 1H NMR, (400 MHz, DMSO-d6): d ¼ 8.83 (s, 1H), 8.37
(s, 1H), 8.32 (s, 1H), 8.25 (dd, J ¼ 8.8, 1.6 Hz, 1H), 7.77–7.62 (m, 5H),
7.00 (d, J ¼ 8.8 Hz, 2H), 4.40 (t, J ¼ 6.4 Hz, 2H), 3.75 (s, 3H), 1.74 (t, J
2-Cyano-3-(9-octyl-9H-carbazol-3-yl)-acrylic acid (CC).
Product: yellow colored solid, yield: 59%. Melting point: 154
ꢁC 1H NMR, (400 MHz, CDCl3-d): d ¼ 8.78 (s, 1H), 8.48 (s, 1H),
8.26 (d, J ¼ 8.4 Hz, 1H), 8.16 (d, J ¼ 7.6 Hz, 1H), 7.54 (t, J ¼ 8
Hz, 1H), 7.47 (t, J ¼ 8.8 Hz, 2H), 7.34 (t, J ¼ 7.6 Hz, 1H), 4.33 (t,
J ¼ 7.2 Hz, 2H), 1.89 (p, J ¼ 7.3 Hz, 2H), 1.41–1.21 (m, 10H),
0.86 (3, J ¼ 6.4 Hz, 3H) ppm. 13C NMR, (100 MHz, CDCl3-d):
d ¼ 168.99, 157.42, 143.68, 141.11, 129.54, 127.04, 125.86,
123.59, 122.73, 122.32, 120.96, 120.75, 116.54, 109.53, 109.47,
96.34, 43.47, 31.76, 29.30, 29.14, 28.94, 27.25, 22.60, 14.06
ppm. HRMS (ESI) m/z: calculated for C24H26N2O2 [M ꢀ H]+:
374.20, found: 373.17.
¼ 6.4 Hz, 2H), 1.20–1.11 (m, 10H), 0.75 (t, J ¼ 8.4 Hz, 3H) ppm. 13
C
NMR, (100 MHz, DMSO-d6): d ¼ 164.14, 158.43, 155.07, 143.03,
139.73, 133.05, 132.53, 127.74, 127.51, 125.99, 125.49, 122.61,
122.28, 117.93, 117.25, 114.33, 110.45, 110.31, 98.28, 55.09, 42.63,
31.10, 28.63, 28.53, 28.46, 26.31, 21.95, 13.82 ppm. HRMS (ESI) m/z:
calculated for C31H32N2O3 [M ꢀ H]+: 480.24, found: 479.42.
{5-[6-(4-Methoxy-phenyl)-9-octyl-9H-carbazol-3-ylmethylene]-
4-oxo-2-thioxo-thiazolidin-3-yl}-acetic acid (OMR). Product:
brick red colored solid, yield: 92%. Melting point: 110 ꢁC 1H
NMR, (400 MHz, DMSO-d6): d ¼ 8.48 (d, J ¼ 10.8 Hz, 2H), 7.96 (s,
1H), 7.73–7.63 (m, 6H), 7.00 (d, J ¼ 8.8 Hz, 2H), 4.66 (s, 2H), 4.38
(s, 2H), 3.75 (s, 3H), 1.73–1.71 (m, 2H), 1.20–1.11 (m, 10H), 0.74
(t, J ¼ 6.8 Hz, 3H) ppm. MS (ESI) m/z: calculated for
3-{6-[Bis-(4-methoxy-phenyl)-amino]-9-octyl-9H-carbazol-3-yl}-
1
2-cyano-acrylic acid (OMNC). Yield: 66.6%. H NMR (400 MHz,
CDCl3-d): d 8.44 (d, J ¼ 6.3 Hz, 2H), 8.40 (d, J ¼ 1.6 Hz, 1H), 7.79
(d, J ¼ 1.8 Hz, 1H), 7.46 (d, J ¼ 8.8 Hz, 1H), 7.37–7.29 (m, 2H),
7.10–7.00 (m, 4H), 6.88–6.79 (m, 4H), 4.31 (t, J ¼ 7.1 Hz, 2H), 3.83
(s, 6H), 1.96–1.87 (m, 2H), 1.38–1.28 (m, 10H), 0.90 (d, J ¼ 6.6 Hz,
3H). 13C NMR (101 MHz, CDCl3-d): d ¼ 168.09, 157.32, 155.06,
143.97, 142.89, 142.21, 136.90, 128.13, 127.43, 124.96, 124.38,
123.62, 123.19, 122.19, 116.68, 115.81, 114.66, 110.15, 109.67,
96.23, 55.55, 43.63, 31.85, 29.72, 29.52, 29.48, 29.36, 29.27, 29.06,
28.98, 27.26, 22.67, 14.15, 14.13.
3-{6-[Bis-(4-hexyloxy-phenyl)-amino]-9-octyl-9H-carbazol-3-yl}-
2-cyano-acrylic acid (CNC). Yield: 68.5%. 1H NMR (400 MHz,
CDCl3-d): d 8.42 (s, 3H), 7.78 (s, 1H), 7.46 (d, J ¼ 9.3 Hz, 1H), 7.30
(d, J ¼ 6.9 Hz, 2H), 7.04 (d, J ¼ 8.7 Hz, 4H), 6.83 (d, J ¼ 8.8 Hz,
C
33H34N2O4S2 [M ꢀ H]+: 586.20, found: 585.25.
2-Cyano-3-(9-octyl-6-phenyl-9H-carbazol-3-yl)-acrylic acid (PC).
ꢁ
Product: yellow colored solid, yield: 92%. Melting point: 204 C
1H NMR, (400 MHz, DMSO-d6): d ¼ 8.84 (s, 1H), 8.38 (d, J ¼ 4.4
Hz, 2H), 8.25 (d, J ¼ 8.4 Hz, 1H), 7.79–7.76 (m, 2H), 7.71 (d, J ¼ 8
Hz, 3H), 7.44 (t, J ¼ 7.6 Hz, 2H), 7.30 (t, J ¼ 7.2 Hz, 1H), 4.42 (t, J ¼
6.8 Hz, 2H), 1.74–1.72 (m, 2H), 1.20–1.11 (m, 10H), 0.74 (t, J ¼ 6.4
Hz, 3H) ppm. 13C NMR, (100 MHz, DMSO-d6): d ¼ 164.11, 155.04,
143.05, 140.63, 140.14, 132.71, 128.88, 127.58, 126.71, 125.97,
125.81, 122.61, 122.38, 118.53, 117.22, 110.52, 110.34, 98.43,
42.66, 31.10, 28.63, 28.53, 28.45, 26.31, 21.94, 13.81 ppm. MS (ESI)
m/z: calculated for C30H30N2O2 [M + H]+: 450.23, found: 449.17.
37352 | RSC Adv., 2016, 6, 37347–37361
This journal is © The Royal Society of Chemistry 2016

View Article Online
Paper
RSC Advances
Fig. 2 Normalised absorption spectrum of TBC, TMC, OMC, PC and CC dyes in CHCl3 solution and on TiO2 films.
4H), 4.31 (t, J ¼ 7.0 Hz, 2H), 3.96 (t, J ¼ 6.5 Hz, 4H), 2.39 (s, 2H), 8.04 (d, J ¼ 1.9 Hz, 1H), 7.59–7.35 (m, 8H), 7.31 (s, 1H), 7.17 (d, J
2.04 (s, 2H), 1.80 (s, 2H), 1.35 (dd, J ¼ 13.0, 6.0 Hz, 22H), 0.95– ¼ 8.7 Hz, 4H), 6.57 (dd, J ¼ 6.6, 2.3 Hz, 4H), 4.35 (s, 2H), 4.00 (dt,
0.90 (m, 9H). 13C NMR (101 MHz, CDCl3): d ¼ 179.43, 168.59, J ¼ 9.5, 6.6 Hz, 8H), 1.87–1.70 (m, 10H), 1.44–1.31 (m, 34H),
157.31, 154.68, 143.98, 143.02, 142.02, 136.82, 130.03, 129.73, 0.95–0.87 (m, 15H). 13C NMR (101 MHz, CDCl3-d): d ¼ 179.48,
128.01, 127.53, 124.99, 124.30, 123.60, 123.22, 122.13, 116.59, 168.39, 159.43, 157.24, 156.96, 146.61, 144.03, 141.68, 137.87,
115.64, 115.25, 110.10, 109.67, 96.18, 77.33, 77.01, 76.69, 68.32, 132.04, 130.83, 130.11, 129.75, 127.99, 127.68, 126.57, 123.79,
43.63, 33.93, 31.93, 31.85, 31.63, 29.70, 29.68, 29.65, 29.60, 29.51, 123.24, 123.13, 122.35, 118.68, 110.33, 109.75, 105.28, 100.42,
29.47, 29.44, 29.38, 29.34, 29.25, 29.15, 29.07, 27.26, 25.79, 24.70, 77.36, 77.24, 77.04, 76.72, 68.40, 68.12, 43.70, 33.95, 31.95,
22.70, 22.66, 22.63, 14.13, 14.11, 14.06.
31.87, 31.64, 31.46, 29.72, 29.62, 29.54, 29.50, 29.38, 29.34,
6-[Bis(20,40-bis(hexyloxy)-[1,10-biphenyl]-4-yl)amino]-9-octyl- 29.28, 29.09, 27.31, 27.24, 25.79, 25.76, 24.72, 22.72, 22.68,
9H-carbazol-3-yl}-2-cyano-acrylic acid (HNC). Yield: 67.6%. 1H 22.65, 22.58, 14.13, 14.08, 14.04, 13.67.
NMR (400 MHz, CDCl3-d): d 8.48 (s, 1H), 8.45 (d, J ¼ 7.2 Hz, 2H),
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 37347–37361 | 37353

View Article Online
RSC Advances
Paper
5.2. Optical properties
5. Results and discussion
Fig. 1A and B shows the absorption spectra of phenyl donor
substituted sensitizers in chloroform solvent and the data are
summarized in Table 1. All the sensitizers shows two types of
absorption bands, one at higher energy around 250–360 nm,
which arise from p–p* transition and another band at lower
energy appears at 360–540 nm which can be assigned to intra-
molecular charge transfer transition from electron donating
moiety (CTB, CTM, COMe, CPB) to electron acceptor moiety
(CAA, RA).21 Due to the difference in acceptor groups (CAA or
RA), the intramolecular charge transfer bands occurred in
different wavelength, e.g. 408 to 418 nm for CAA acceptor
groups and 453 to 457 nm for RA acceptor groups (Fig. S1†).
While analyzing the effect of various substituents in the phenyl
donor on carbazole unit, we didn't observe any major difference
in the absorption spectrum, only slight shis are observed. By
varying the donor moiety on carbazole unit the higher energy
band is red shiing in the order of CC (408 nm) < TMC (413 nm)
< PC (414 nm) < TBC (416 nm) < OMC (418 nm) for cyano acrylic
acid substituted carbazole sensitizers and when rhodanine is
used as acceptor, an order of TMR (453 nm) < PR (454 nm) < TBR
(456 nm) < OMR (457 nm) is observed. Among all compounds,
methoxyphenyl incorporated sensitizer (OMC) shows the most
red-shied absorption wavelength. TBR with tert-butyl phenyl
as donating moiety shows bathochromic absorption shi by 3
nm compared to TMC. Due to the introduction of methoxy
groups in para position of phenyl ring, OMC (lmax ¼ 418 nm, 3 ¼
7.66 ꢂ 105 Mꢀ1 cmꢀ1) shows three times higher molar extinc-
tion coefficient than PC (lmax ¼ 414 nm, 3 ¼ 4.36 ꢂ 105 Mꢀ1
cmꢀ1) The molar extinction coefficient of TBC (lmax ¼ 416 nm, 3
5.1. Synthesis and characterization
Schemes 1 and 2 shows the structures of carbazole based
sensitizers having donor fragments with different strength.
The synthetic route of CBAI is shown in Scheme 3. CB is
prepared from carbazole via N-alkylation with 1-bromo octane,
followed by Vilsmeier hack formylation reaction to get CBA.
Iodination of CBA using KI and KIO3 afforded CBAI. Four
different types of aryl boronic acid (4-tert-butylphenylboronic
acid,
(2,4,6-trimethylphenyl)boronic
acid,
4-methox-
yphenylboronic acid and 4-phenylboronic acid) were coupled
with CBAI via Suzuki coupling reaction to get their corre-
sponding aldehydes. A peak around 10.09–10.1 ppm in proton
NMR clearly shows the presence of the aldehyde group.
Knoevenagel condensation of aldehydes precursors with cyano
acetic acid or rhodanine acetic acid attains the target
compounds as shown in Scheme 4. CBAI undergoes Buchwald
coupling with three different types of aryl amines to afford the
respective aldehydes followed by Knoevenagel condensation to
obtain nal compounds as depicted in Scheme 5. Similarly
CBA (absence of donating group) undergo Knoevenagel
condensation with cyano acetic acid in the presence of
ammonium acetate and acetic acid to give CC. These new
molecules are characterized by 1H, 13C NMR, melting point
measurement and mass spectroscopy techniques.
¼ 2.66 ꢂ 105 Mꢀ1 cmꢀ1) are slightly higher than TMC (lmax
¼
413 nm, 3 ¼ 1.97 ꢂ 105 Mꢀ1 cmꢀ1) and CC (lmax ¼ 408 nm, 3 ¼
1.70 ꢂ 105 Mꢀ1 cmꢀ1). RA derivatives shows molar extinction
coefficient in the range of (1.66 to 3.23 ꢂ 105 Mꢀ1 cmꢀ1). It is
worth noting that inspite of simple structures of these sensi-
tizers, their molar extinction coefficient are signicantly higher
than those of ruthenium sensitizers.22
The absorption behavior of the sensitizers attached to the
TiO2 surface is depicted in Fig. 2. Those absorption spectra are
broadened and blue shied as compared in solution state due
to the strong interaction between dyes and semiconductor
surface. Hypsochromic shi of 5 nm to 42 nm for cyano acrylic
acid based compounds (OMC, PC, TBC, CC and TMC) occurred
due to the formation of H type aggregation or deprotonation of
the dyes on the surface of the TiO2 lms.17,23 Rhodanine acetic
Fig.
3 Normalised absorption spectrum of amine donor based
carbazole dyes in CHCl3 solutions.
Table 2 Absorption, emission and electrochemical properties of amine donor based carbazole dyes
lmaxa (nm)/[3 (Mꢀ1 cmꢀ1)]
lemia (nm)
Eoxb (V) vs. NHE
E0–0 (eV) vs. NHE
Eox–E0–0 (V)
c
OMNC
CNC
HNC
382 (4560) 455 (1560)
382 (17 900) 459 (5940)
331 (67 200) 387 (39 300)
493
618
513
0.73
1.18
0.83
2.81
2.92
2.75
ꢀ2.08
ꢀ1.74
ꢀ1.92
a
Absorption and emission spectrum of amine donor based carbazole dyes were measured in CHCl3 solutions. b Cyclic voltammograms of the rst
oxidation potential of the dyes were measured in dichloromethane solution containing 0.1 M TBAPF6 used as an supporting electrolyte (working
electrode: Pt, counter electrode: Pt wire, reference electrode: Ag/AgCl calibrated with ferrocene/ferrocenium (Fc/Fc+) as an internal reference and
converted to NHE by adding 630 mV). c The E0–0 transition values was estimated from the onset of absorption and emission spectra.
37354 | RSC Adv., 2016, 6, 37347–37361
This journal is © The Royal Society of Chemistry 2016

View Article Online
Paper
RSC Advances
Fig. 4 Normalised fluorescence spectrum of phenyl donor based carbazole dyes in CHCl3 solutions.
values which appear around 570 nm. Among sensitizers with
phenyl donors the onset values appeared around 490 nm for
CAA anchoring group and 530 nm for RA anchoring unit. The
maximum absorption coefficient of the amine donor based
sensitizers are in the order of HNC (3 ¼ 3.93 ꢂ 104 Mꢀ1 cmꢀ1 at
387 nm) > CNC (3 ¼ 5.94 ꢂ 103 Mꢀ1 cmꢀ1 at 459 nm) > OMNC (3
¼ 1.56 ꢂ 103 Mꢀ1 cmꢀ1 at 455 nm). Increasing bulkiness via
alkylation in the donor fragment prevents the dye aggregation
which increases their molar absorption coefficient. The emis-
sion spectra of all carbazole sensitizers are recorded in chloro-
form and displayed in Fig. 4A and B. The emission data are
collected in Table 1 and the values of different donor
compounds are appeared in the same order of absorption
maximum. Fluorescence spectra of amine donor dyes are dis-
played in Fig. 5 and the data is given in Table 2. When we look
over the absorption spectra, a similar maximum absorption
peak is observed for OMNC and CNC. However, the maximum
emission peak of CNC is observed at 618 nm which is 125 nm
higher than OMNC (493 nm). This reveals that the presence of
alkyl chains in the donor fragment have signicant impact in
changing the excited state of the molecule.
Fig. 5 Normalised fluorescence spectrum of amine donor based
carbazole dyes in CHCl3 solutions.
acid based dyes (TBR, TMR, OMR and PR) shows more prom-
inent blue shi by ꢃ86 nm owing to the strong deprotonation of
carboxylic acid onto the TiO2 surface (Fig. S2†). While
comparing these two acceptor moieties, rhodanine acetic acid
substituted compounds absorb at longer wavelength than that
of cyano acrylic acid based compounds both in chloroform
solution and TiO2 lm.
To further increase the donor strength, we choose phenyl
amine based donor groups. Aryl amine donors with different
alkyl chain substitution were synthesized to afford OMNC, CNC
and HNC. Fig. 3 shows the absorption spectra of OMNC, CNC
and HNC and the data is displayed in Table 2. Firstly, OMNC
with para-substituted methoxy groups on diphenylamine shows
the maximum absorption at 382 and 455 nm. By increasing the
alkyl chain from methoxy to hexyloxy, a slightly red shied
absorption is observed at 382 and 459 nm. However, by intro-
ducing one more phenyl group with two hexyloxy groups
attached in ortho and para positions leads to blue shied
absorption at 331 nm, which indicates the decreased donating
strength of amine donor. Replacing phenyl donors with
stronger amine donor results in red shied onset absorption
5.3. Electrochemical properties
The rst oxidation potential of the carbazole based sensitizers
corresponds to the ground state potential level (i.e. HOMO) of
the dye and is important for their photovoltaic applications.
Cyclic voltammetry measurements were carried out to analyze
the HOMO level of the dyes using tetrabutyl ammonium hexa-
uoro phosphate (TBAPF6) as the supporting electrolyte.
Ferrocene/ferrocenium (Fc/Fc+) is used as an internal standard.
The electrochemical data of all carbazole sensitizers are pre-
sented in Fig. 6 and summarized in Table 1. Single oxidation
peaks are observed for CC, PC, PR and TMR and two oxidation
peaks are observed for OMC, OMR, TBR and TMC and three
oxidation peaks is observed for TBC. The rst oxidation peaks of
all dyes and second oxidation peak of OMC, OMR and TMC are
reversible in nature, which suggest the oxidized states of the
dyes are quite stable.24 PC, TBR and TMR display quasi-
reversible second oxidation peaks. The rst oxidation peaks of
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 37347–37361 | 37355

View Article Online
RSC Advances
Paper
Fig. 6 Cyclic voltammograms of the phenyl donor based carbazole dyes in acetonitrile solutions.
all dyes occurs in the range of 1.15–1.58 V, which corresponds to donor dyes which is due to the strong donating ability of amine
the removal of electron from the donor moiety and second donor which results in easy removal of electrons and oxidise at
oxidation peaks occur at higher potentials, which can be lower potential. The HOMO level of the dyes are more positive
attributed to the removal of electron from conjugated backbone than Iꢀ/I3ꢀ electrolyte (0.4 V vs. NHE), which reveals the efficient
of the dyes.25 The oxidation potential of the sensitizers with dye regeneration from electrolytes.
amine donor in dichloromethane solutions are depicted in
The excited state oxidation potential (i.e. LUMO) can be
Fig. 7 and Table 2. The rst oxidation potentials of OMNC, CNC calculated by subtracting zero–zero excitation energy (E0–0) from
and HNC are 0.73 V, 1.18 V and 0.83 V, respectively. All the HOMO level of the dyes. The E0–0 energy values can be obtained
oxidation peaks are reversible in nature, showing that the from the point of intersection of the experimental absorption
oxidized states of these dyes are quite stable. The HOMO of the and emission spectra of sensitizers and the data is displayed in
amine donor dyes occurs at lower potential than the phenyl Table 1. The obtained LUMO values are sufficient for electron
37356 | RSC Adv., 2016, 6, 37347–37361
This journal is © The Royal Society of Chemistry 2016

View Article Online
Paper
RSC Advances
Fig. 7 Cyclic voltammograms of the phenyl donor based carbazole dyes in dichloromethane solutions.
injection into conduction band potential of TiO2 (ꢀ0.5 V vs. Among the sensitizers with phenyl based donors, OMC with
NHE). The electrochemical data supports the thermodynamic methoxy substituted phenyl and maximum absorption wave-
driving force for efficient dye regeneration and electron injec- length shows higher JSC of 4.96 mA cmꢀ2 than that (4.15 mA
tion in DSSC (Fig. 8).
cmꢀ2) of tert-butyl phenyl substituted carbazole (TBC). For other
The geometries of the dyes are optimized by density func- types of substitution on the phenyl group, the JSC only turn out
tional theory (DFT) calculations at the B3LYP.6-31G(d)21 level to to be around 2 to 2.6 mA cmꢀ2. Among phenyl based donors,
analyze further the distribution of HOMO and lowest unoccu- OMC and TBC have higher VOC of 730 mV. The VOC of OMC and
pied molecular orbital (LUMO) energy levels of the sensitizers. TBC are higher, because the presence of methoxy and tert-butyl
Fig. S3A–F and Table S1† display the electron distributions of groups which suppresses the recombination of injected elec-
the HOMO and LUMO of the dyes. HOMOs are mainly delo- trons by the electrolyte. Overall OMC shows higher efficiency of
calized in the phenyl and carbazole groups in the phenyl based 2.69% and 2.17% for TBC while other shows in the range of 1%.
donor fragment, while it resides mainly on the amino substit- These results show that varying the substituents in the phenyl
uents in the amine based donor group. LUMOs show localized group has signicant impact on the overall conversion effi-
electron distributions on the acceptor parts cyanoacrylic acid ciencies. The presence of trimethylphenyl (TMC) and phenyl
and rhodanine acetic acid. The observed spatially separated (PC) donors shows similar efficiency (JSC ¼ 2.06 mA cmꢀ2, VOC
¼
frontier orbitals strongly promote intramolecular charge sepa- 0.634 V, ff ¼ 0.750, h ¼ 0.98%) for TMC and (JSC ¼ 2.24 mA
ration and hence favor efficient electron injection from the cmꢀ2, VOC ¼ 0.574 V, ff ¼ 0.765, h ¼ 0.98%) for PC. Interestingly,
excited state of the dye into the semiconducting oxide.
the JSC of CC (no donor group) is 2.63 mA cmꢀ2 with the effi-
ciency of 1.11% which is higher than the substituted carbazole
TMC and PC. Further comparison of the sensitizers with cyano
acrylic acid and rhodanine acetic acid as acceptors, the later
5.4. Photovoltaic properties
5.4.1. IV characterization. The impact of different donors shows low short circuit current density. For instance, 4.96 mA
on solar energy conversion is analyzed by employing the cmꢀ2 OMC is reduced to 2.50 mA cmꢀ2 for OMR. This lower JSC
sensitizers in DSSC with iodine iodide redox electrolyte. The of RA acceptor is due to its inefficient electron injection to the
photocurrent density–photovoltage (J–V) curves of sensitizers TiO2. Due to the lower JSC, all the RA substituted compounds
with phenyl and amine based donors measured under simu- shows lower efficiency. Despite this low efficiency, RA
lated AM 1.5 solar irradiation (100 mW cmꢀ2) are depicted in substituted compounds shows higher ll factor among all the
Fig. 9A and B. The parameters such as short circuit current (JSC), compounds. As expected increasing the strength of the donor
open circuit voltage (VOC), ll factor (ff) and photovoltaic from phenyl to amine leads to higher short circuit current
conversion efficiency (h) data are collected in Tables 3 & 4. density and power conversion efficiency. For example
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 37347–37361 | 37357

View Article Online
RSC Advances
Paper
ꢀ
Fig. 8 Energy band gap diagrams for all the dyes in comparison to TiO2 and Iꢀ/I3
.
sensitizers with dianisole amine as donor shows ꢃ2 mA higher reveal that the capacitance of lengthy alkyl chain substituted
JSC than OMC with only methoxy substitution. Among the CNC and HNC is slightly shied to higher VOC than the methyl
different substituents in the amine donor, the hexyloxy and substituted OMNC. However the observed difference in the
methoxy substituted compounds shows higher short circuit conduction band of TiO2 is small and suggests that this factor
current and higher efficiency of 3.3% and 3.25% respectively. has minimal role in determining the open circuit potential of
The bulky amine donor leads to lesser JSC than the other which these devices. Hence, the recombination factor should be
may be due to its lower absorbance. However changes in the responsible for the observed difference in the VOC. Electron
substituents of the amine donor have greater impact on the
open circuit potential. VOC is obtained in the order of HNC (752
mV) > CNC (735 mV) > OMNC (690 mV). Changing the methoxy
substituent to hexyloxy, an increase in VOC of 45 mV is achieved
and at the same time a higher efficiency of 3.33%. Introducing
more alkoxy and phenyl group in the amine donor (HNC)
resulted in highest VOC value among all carbazole dyes. This is
due to the presence of more alkoxy chains in HNC helps in
restricting the electrolyte to reach the TiO2 surface and thus
preventing the recombination. Despite the low VOC of OMNC
than CNC, the higher ll factor of OMNC results in similar
efficiency of 3.25%. Although HNC shows higher ll factor than
other amine donors, its lower JSC resulted in overall conversion
efficiency of 3.01%.
In Fig. 9b, the dark current curves of OMNC, CNC and HNC
are displayed. The dark current values of dyes are in the order of
HNC < OMNC ꢃ CNC indicating that, on increase the VOC are
benecial to suppress the recombination of injected electrons
at the interface or in the electrolyte.
5.4.2. Transient measurements. Transient photocurrent
and photovoltage measurements were carried out to get further
insights into the difference in the open circuit potential of the
dyes OMNC, CNC and HNC which gives vital information about
the effect of the alkyl chain substituents on the donor fragment.
Variation in open circuit potential can possibly originate from
three factors: (a) the shi in the conduction band of the TiO2
semiconductor which alters the density of occupied states (dos),
(b) recombination of the injected electrons in the semi-
conductor to the redox mediator, and (c) the redox potential of
the electrolyte species. Since the same redox mediator is used in
all these dyes, we neglect this factor (c) from the discussion.2,26,27
Measurement of capacitance as a function of voltage will
provide information about the conduction band of the TiO2 and
the plot is depicted Fig. 10a. The results from this measurement based on carbazole sensitizers.
Fig. 9 J–V curves of DSSCs (A) phenyl donor and (B) amine donor
37358 | RSC Adv., 2016, 6, 37347–37361
This journal is © The Royal Society of Chemistry 2016

View Article Online
Paper
RSC Advances
Table 3 J–V curves of DSSC sensitized with phenyl donor based
carbazole dyes in iodine electrolytea
JSC (mA cmꢀ2
)
VOC (V)
ff
h (%)
TBC
TMC
OMC
PC
4.15
2.06
4.96
2.24
2.63
1.40
0.813
2.50
0.730
0.634
0.730
0.574
0.577
0.537
0.510
0.525
0.718
0.750
0.743
0.765
0.731
0.775
0.779
0.775
2.17
0.98
2.69
0.98
1.11
0.58
0.32
1.02
CC
TBR
TMR
OMR
a
TiO2 lm has a 8 mm scattering layer and a 5 mm transparent layer.
DMII (1,3-dimethylimidazolium
Electrolyte composition: 1.0
M
iodide), 0.03 M iodine, 0.025 M NaI, 0.5 M TBP, 0.1 M guanidinium
thiocyanide and acetonitrile solvent.
Fig. 11 IPCE spectra of DSSCs based on OMC, OMNC, CNC and HNC.
recombination occurring at the TiO2 electrolyte interface can be
revealed by the measurement of lifetime as a function of VOC
and the plot is displayed in Fig. 10b. At a given VOC, the lifetime
of alkyl chain substituted HNC and CNC is higher than the
OMNC implying the slower recombination rate of the former
dyes. This observation further supports the presence of bulki-
ness in the donor group of HNC prevents the electrolyte to reach
the TiO2 surface that assist in reducing the recombination and
achieving higher VOC. Lower lifetime of OMNC indicates its
higher recombination rate that results in lower VOC. The
observed trend is consistent with the observed VOC of the
devices. Transient measurements reveal that recombination is
critical factor in determining the open circuit potential which
found to be affected by the bulkiness in the donor fragment.
5.4.3. IPCE measurements. Based on the performance of J–
V curves, the IPCE curves are recorded for higher JSC dyes such
as OMC, OMNC, CNC and HNC and are displayed in Fig. 11.
Similar to the absorption spectra of dyes, the IPCE response
shows two bands in the broad range of 340–660 nm. The rst
band of IPCE based on all dyes exceeds 70% (350–400 nm) and
the next band exceeds 50% (400–520 nm). The IPCE values
exceeds 50% in the range of 340–485 nm for OMC, which rea-
ches it maximum of 95% at 378 nm and of 72% at 450 nm. The
maximum IPCE of OMNC, CNC and HNC at 460 nm are 64%,
62% and 56%, respectively. The onset of the IPCE spectra of
OMC appears at 567 nm, OMNC at 660 nm, CNC at 646 nm and
HNC at 622 nm. The change in IPCE behaviour of the dyes is in
agreement with the absorbance behaviour. The IPCE value of
OMNC are little higher than the CNC, revealing that methoxy
substituted amine donor are strongly converting incident
photon to current than the hexyloxy donor groups. Although,
OMC (without amine donor) shows higher IPCE than OMNC
(with amine donor) the former didn't cover the full spectral
Table 4 Photovoltaic parameters of DSSCs of amine donor based
carbazole sensitizers
JSC (mA cmꢀ2
)
VOC (V)
ff
h (%)
OMNC
CNC
HNC
6.84
6.84
5.43
0.690
0.735
0.752
0.689
0.663
0.737
3.25
3.33
3.01
Fig. 10 (A) Chemical capacitance (B) electron lifetime as function of VOC obtained through transient photocurrent and photovoltage
measurements of devices made with OMNC, CNC and HNC.
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 37347–37361 | 37359

View Article Online
RSC Advances
Paper
region. This may be the reason for the lower performance in no. 604032 of the MESO project. The present study was per-
efficiency of OMC. The overall results suggest that the change in formed within the Project “Establishment of the Laboratory
the donor groups have signicant impact in the IPCE of the D– Photoactive Nanocomposite Materials” No. 14.Z50.31.0016
p–A dyes.
supported by a Mega-grant of the Government of the Russian
Federation. Authors also thank UGC/DST-FIST for NMR facili-
ties in the School of Chemistry, Bharathidasan University.
6. Conclusions
In conclusion, to understand the inuence of donor groups on
the optoelectronic properties, a series of carbazole based
sensitizers with phenyl and amine based donor groups were
successfully synthesized and characterized by 1H and 13C NMR,
and mass spectrometric measurements. For phenyl based
carbazole sensitizers, the maximum absorption appeared
around 418 nm for methoxy substituted phenyl based donor
(OMC) with CAA acceptor and 457 nm for RA as acceptor (OMR).
In the presence of amine donor, CNC shows maximum
absorption at 459 nm which reveals the more donating strength
of amine donor substituted carbazole compounds than the
phenyl donor based sensitizers. Due to H type aggregation of
phenyl based carbazole dyes on the surface of TiO2, a hyp-
sochromic shi is observed in the absorption spectrum of solid
state lms. From electrochemical data, the observed HOMO
and LUMO values supports the feasible thermodynamic driving
force for efficient dye regeneration and electron injection in
DSSC. The DSSC made with these carbazole sensitizers dis-
played overall conversion efficiency in the range of 0.32–3.33%.
OMC shows overall conversion efficiency of 2.69% (JSC ¼ 4.96
mA cmꢀ2, VOC ¼ 0.730 mV, ff ¼ 0.743) for phenyl based donor
groups and CNC shows 3.33% (JSC ¼ 6.84 mA cmꢀ2, VOC ¼ 735
mV, ff ¼ 0.663) for amine based donor groups. RA acceptor
based sensitizers shows overall poor performance due to inef-
cient electron injection of RA acceptor groups to TiO2. The
maximum IPCE of OMNC, CNC and HNC at 460 nm are 64%,
62% and 56%, respectively. Despite the high IPCE (72% at 450
References
¨
1 B. O'Regan and M. Gratzel, Nature, 1991, 353, 737–740.
2 A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo and H. Pettersson,
Chem. Rev., 2010, 110, 6595–6663.
3 S. Mathew, A. Yella, P. Gao, R. Humphry-Baker,
F. E. CurchodBasile, N. Ashari-Astani, I. Tavernelli,
U. Rothlisberger, Md. K. Nazeeruddin and M. Gratzel, Nat.
€
Chem., 2014, 6, 242–247.
4 (a) Z. S. Wang, Y. Cui, K. Hara, Y. Dan-oh, C. Kasada and
A. Shinpo, Adv. Mater., 2007, 19, 1138–1141; (b) K. Hara,
M. Kurashige, Y. Danoh, C. Kasada, A. Shinpo, S. Suga,
K. Sayama and H. Arakawa, New J. Chem., 2003, 27, 783–785.
5 B. Liu, W. Zhu, Q. Zhang, W. Wu, M. Xu, Z. Ning, Y. Xie and
H. Tian, Chem. Commun., 2009, 13, 1766–1768.
6 C. Zafer, M. Kus, G. Turkmen, H. Dincalp, S. Demic,
B. Kuban, Y. Teoman and S. Icli, Sol. Energy Mater. Sol.
Cells, 2007, 91, 427–431.
7 X. Ma, J. Hua, W. Wu, Y. Jin, F. Meng, W. Zhan and H. Tian, A
high-efficiency cyanine dye for dye-sensitized solar cells,
Tetrahedron, 2008, 64, 345–350.
8 S. Hayashi, M. Tanaka, H. Hayashi, S. Eu, T. Umeyama,
Y. Matano, Y. Araki and H. Imahori, J. Phys. Chem. C, 2008,
112, 15576–15585.
9 P. Shen, Y. Liu, X. Huang, B. Zhao, N. Xiang, J. Fei, L. Liu,
X. Wang, H. Huang and S. T. Tan, Dyes Pigm., 2009, 83,
187–197.
nm) of OMC than amine donor compounds, its lower absor- 10 D. Liu, B. Zhao, P. Shen, H. Huang, L. Liu and S. T. Tan, Sci.
bance region than amine based donor compounds results in
China, Ser. B: Chem., 2009, 52, 1198–1209.
lower efficiency. The open circuit potential is found to be 11 F. Sanda, T. Nakai, N. Kobayashi and T. Masuda,
depending on the bulkiness in the amine based donors which Macromolecules, 2004, 37, 2703–2708.
was probed through transient photovoltage measurements. The 12 (a) V. Promarak, A. Punkvuang, T. Sudyoadsuk,
observed results provides vital information for future
construction of donor fragments with more p-conjugated
molecules to achieve wide range of absorption in the visible
region. Overall, the results emphasize the varying strength and
S. Jungsuttiwong, S. Saengsuwan, T. Keawin and
K. Sirithip, Tetrahedron, 2007, 63, 8881–8890; (b)
S. Jagadeeswari, G. Paramaguru, S. Thennarasu and
R. Renganathan, J. Mol. Struct., 2014, 1060, 191–196.
size of the donor fragment in the D–p–A sensitizers have 13 Z. S. Wang, N. Koumura, Y. Cui, M. Takahashi, H. Sekiguchi,
signicant impact in determining the optoelectronic and
photovoltaic properties.
A. Mori, T. Kubo, A. Furube and K. Hara, Chem. Mater., 2008,
20, 3993–4003.
14 S. Cai, X. Hu, J. Han, Z. Zhang, X. Li, C. Wang and J. Su,
Tetrahedron, 2013, 69, 1970–1977.
15 M. Liangwab and J. Chen, Chem. Soc. Rev., 2013, 42, 3453–
3488.
Acknowledgements
S. J. (Ref. No. 039680/E15/2011 Dt: 17.02.2011) thanks UGC-BSR
(RFSMS) for the Fellowship. G. P. thanks the Swiss government 16 (a) M. D. Zhang, H. X. Xie, X. H. Ju, L. Qin, Q. X. Yang,
scholarship (Ref No. 2012.0795/India/OP). R. R. thanks DST-NM
(Ref. No. SR/NM/NS-26/2013(G), DT: 21.10.2014) and UGC
(MRP-MAJOR-CHEM-2013-35169) for the Project and UGC-
Emeritus fellowship (UGC-EF-7855, 2016-2017). P.G. thank
acknowledge funding from the European Union Seventh
Framework Programme [FP7/2007-2013] under grant agreement
H. G. Zheng and X. F. Zhou, Phys. Chem. Chem. Phys.,
2013, 15, 634–641; (b) T. Sudyoadsuk, S. Pansay, S. Morada,
R.
Rattanawan,
S.
Namuangruk,
T.
Kaewin,
S. Jungsuttiwong and V. Promarak, Eur. J. Org. Chem.,
2013, 5051–5063; (c) N. Koumura, Z. S. Wang,
M. Miyashita, Y. Uemura, H. Sekiguchi, Y. Cui, A. Mori,
37360 | RSC Adv., 2016, 6, 37347–37361
This journal is © The Royal Society of Chemistry 2016

View Article Online
Paper
S. Mori and K. Hara, J. Mater. Chem., 2009, 19, 4829–4836; (d)
RSC Advances
44; (e) G. Paramaguru, R. Vijay Solomon, S. Jagadeeswari,
P. Venuvanalinga and R. Renganathan, Eur. J. Org. Chem.,
2014, 753–766; (f) G. Paramaguru, A. Chandiran, P. Gao,
S. Cai, G. Tian, X. Li, J. Su and H. Tian, J. Mater. Chem. A,
2013, 1, 11295–11305; (e) A. Venkateswararao, K. R. Justin
Thomas, C.-T. Li and K.-C. Ho, RSC Adv., 2015, 5, 17953–
17966.
€
R. Rajalingam, M. Gratzel and Md. K. Nazeeruddin, J. Phys.
Chem. C, 2014, 118, 16896–16903.
17 H. Tian, X. C. Yang, R. K. Chen, R. Zhang, A. Hagfeldt and 22 M. K. Nazeeruddin, A. K. Rodicio, R. Humpbry-Baker,
¨
¨
L. Sun, J. Phys. Chem. C, 2008, 112, 11023–11033.
18 B. C. O'Regan and F. J. Lenzmann, J. Phys. Chem. B, 2004,
108, 4342.
E. Muller, P. Liska, N. Vlachopoulos and M. Gratzel, J. Am.
Chem. Soc., 1993, 115, 6382–6390.
23 (a) A. Baheti, P. Singh, C. P. Lee, K. R. Justin Thomas and
K. C. Ho, J. Org. Chem., 2011, 76, 4910–4920; (b)
G. Paramaguru, A. Chandiran, P. Gao, R. Rajalingam,
19 P. Gao, Y. J. Kim, J. H. Yum, T. W. Holcombe,
€
M. K. Nazeeruddin and M. Gratzel, J. Mater. Chem. A, 2013,
¨
1, 5535–5544.
M. Gratzel and Md. K. Nazeeruddin, ChemPhysChem, 2015,
€
20 W. H. Nguyen, C. D. Bailie, J. Burschka, T. Moehl, M. Gratzel,
16, 1035–1041.
M. D. McGehee and A. Sellinger, Chem. Mater., 2013, 25, 24 Y. Ooyama, Y. Hagiwara, Y. Oda, T. Mizumo, Y. Harima and
1519–1525. J. Ohshita, New J. Chem., 2013, 37, 2336–2340.
21 (a) K. R. J. Thomas, Y. C. Hsu, J. T. Lin, K. M. Lee, K. C. Ho, 25 (a) P. B. Pati and S. S. Zade, Tetrahedron, 2013, 69, 2167–
C. H. Lai, Y. M. Cheng and P. T. Chou, Chem. Mater., 2008,
20, 1830–1840; (b) J. Song, F. Zhang, C. Li, W. Liu, B. Li,
Y. Huang and Z. Bo, J. Phys. Chem. C, 2009, 113, 13391–
2174; (b) S. Jagadeeswari, G. Paramaguru, R. Madhumitha,
Md. K. Nazeeruddin and R. Renganathan, J. Photochem.
Photobiol., A, 2015, 299, 194–202.
13397; (c) P. G. Hoertz, R. A. Carlisle and G. J. Meyer, Nano 26 L. M. Peter, Phys. Chem. Chem. Phys., 2007, 9, 2630–2642.
Lett., 2003, 3, 325–330; (d) G. Paramaguru, R. Vijay 27 B. C. O'Regan and J. R. Durrant, Acc. Chem. Res., 2009, 42,
Solomon, S. Jagadeeswari, P. Venuvanalingam and
1799–1808.
R. Renganathan, J. Photochem. Photobiol., A, 2013, 271, 31–
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 37347–37361 | 37361